In a communication system, a reception side transmits feedback information of a signal received from a transmission side so as to accomplish efficient communication. The transmitted feedback information will now be briefly described in, for example, a multiple input-multiple output (MIMO) communication.
FIG. 1 is a schematic diagram showing the configurations of transmission and reception sides in a general MIMO communication system and information fed back from the reception side.
First, a scheduler 101 of the transmission side 100 allocates transmission signals to adequate communication resources using the feedback information received from the reception side. Thereafter, the transmission signals are subjected to encoding and mapping 102, MIMO encoding 103 and OFDM modulation 104 and are transmitted through a channel. The reception side 190 receives the transmission signals and performs inverse processes of the processes performed by the transmission side. That is, the reception signals are subjected to OFDM demodulation 105, MIMO decoding 106 and decoding and demapping 107 so as to acquire transmission information. The reception side 190 performs channel estimation 108 using the OFDM-demodulated signals and acquires the feedback information (109) which will be used in MIMO decoding 106 and be fed back to the transmission side. The feedback information configures user information 110 and is fed back to the transmission side 100 as shown in FIG. 1.
The feedback information of the reception side 190 is divided into feedback information related to an orthogonal frequency division multiplexing (OFDM) system and feedback information related to an MIMO system, both of which will now be described.
In the OFDM system, channel quality information (hereinafter, abbreviated to CQI) and channel status information (for example, a signal-to-interference and noise ratio (SINR) or a signal-to-noise ratio) measured by a user equipment (hereinafter, abbreviated to UE) are fed back to a base station. The base station allocates resources and determines the level of modulation and coding scheme on the basis of the feedback information.
A MIMO-OFDM system may be divided into an open loop (hereinafter, abbreviated to OL) mode and a close loop (hereinafter, abbreviated to CL) mode. In the OLMIMO system, the UE reports the same feedback information as the existing OFDM system to the base station. In contrast, in the CL-MIMO system, the UE transmits additional information used in downlink transmission, such as channel information, a weighted value, a precoding matrix index (PMI), and a phase for a CDD method.
A precoded CL-MIMO system will now be described. The UE estimates a PMI which is most suitable for a current channel status. The CQI value varies according to the estimated PMI. In general, the UE selects a PMI, which most increases the CQI, from PMIs applied to the same channel and reports the CQI value to the base station.
In the MIMO system, the number of ranks which are applied according to the status of the channel may be determined. The UE may inform the base station of information indicating how many ranks should be applied in the current channel status, for most efficient transmission.
The MIMO system may be operated by the OL mode and the CL mode as described above. In general, the OL mode is used in a high-speed status and the CL mode is used in a low-speed status. The UE selects one of the OL mode or the CL mode and informs the base station of the mode.
The UE transmits the feedback information in consideration of feedback overhead in a predetermined period. In general, several subcarriers are clustered and computed, rather than measurement of a value corresponding to one subcarrier. Information about a reporting period of the feedback information of the UE and a frequency cluster is not informed from each UE to the base station, that is, it is determined by the base station and is informed to each UE.
All the UEs experience different channel statuses. Therefore, if the base station determines a cluster size and reporting period, which are not suitable for the channel statuses of the UEs, in a state in which the base station does not know the statuses of the UEs, system capability may deteriorate.